This invention relates to olefin polymerization catalysts and particularly relates to specific silane modifier compounds useful in a supported alphaolefin polymerization catalyst system which can produce propylene polymers having a broadened molecular weight distribution.
Use of solid, transition metal-based, olefin polymerization catalyst components is well known in the art including such solid components supported on a metal oxide, halide or other salt such as widely-described magnesium-containing, titanium halide-based catalyst components. Also known is incorporating an electron donor compound into the titanium-containing component. An olefin polymerization system typically comprises a titanium-containing compound, an alkylaluminum compound and an electron donor modifier compound. The electron donor modifier used in combination with the alkyl aluminum compound and solid titanium-containing compound is distinct from the electron donor which may be incorporated within the titanium-containing compound. Many classes of electron donors have been disclosed for possible use as electron donor modifiers used during polymerization.
One class of such electron donor compounds is organosilanes. For example in U.S. Pat. No. 4,540,679, organosilanes, especially aromatic silanes are described. Use of organosilanes as cocatalyst modifiers also is described in Published U.K. Application No. 2,111,066 and U.S. Pat. Nos. 4,442,276, 4,472,524, 4,478,660, and 4,522,930. Other aliphatic and aromatic silanes used in polymerization catalysts are described in U.S. Pat. Nos. 4,420,594, 4,525,555 and 4,565,798.
Although many organosilane modifiers have been described, it is advantageous to tailor a catalyst system to obtain a specific set of properties of a resulting polymer product. For example, in certain applications a product with a broader molecular weight distribution is desirable. Such a product has a lower melt viscosity at high shear rates than a product with a narrower molecular weight distribution. Many polymer fabrication processes which operate with high shear rates, such as injection molding, oriented film, and thermobonded fibers, would benefit with a lower viscosity product by improving throughput rates and reducing energy costs. Products with higher stiffness, as measured by flexural modulus, are important for injection molded, extruded, and film products since the fabricated parts can be downgauged so that less material would be needed to maintain product properties. Also important is maintaining high activity and low atactic levels such as measured by hexane soluble and extractable materials formed during polymerization.
The specific aliphatic silane modifiers of this invention not only are used in supported catalysts to provide high yield and low atactic products, but also produce a polymer with a broader molecular weight distribution than produced using the preferred silane described in U.S. Pat. No. 4,829,038, incorporated by reference herein, having a common inventorship to this application. Comparative data show a silane of this invention produces a better balance of properties--yield, atactic level, and molecular weight distribution--than other silanes having similar substituent groups.